Control of the radial electric field shear by modification of the magnetic field configuration in LHD

Full text: Control of the radial electric field, Er, is considered to be important in helical plasmas, because the radial electric field and its shear are expected to reduce neoclassical and anomalous transport, respectively. In general, positive or negative electric field have been obtained by decreasing or increasing the electron density. Although the sign of the radial electric field can be controlled by changing the collisionality, modification of the magnetic field is required to achieve further control of the radial electric field, especially producing a strong radial electric field shear. In the Large Helical Device (LHD) the radial electric field profiles are shown to be controlled by the modification of the magnetic field by 1) changing the radial profile of the helical ripples, 2) creating a magnetic island with an external perturbation field coil and 3) changing the local island divertor coil current. Strong shear of the radial electric field is demonstrated to be produced inside the plasma by controlling the helical ripple, magnetic topology (magnetic island and LID configuration) to trigger an internal transport barrier and reduce impurity influx. (author)